Pressurized water nuclear reactors comprise, inside a vessel, the core of the nuclear reactor consisting of assemblies of prismatic shape arranged with their axis vertical and, above the core, upper internals comprising in particular a set of vertical guide tubes making it possible to guide control rods consisting of bundles of rods extending parallel to one another and enclosing a material which absorbs neutrons.
The guide tubes of the upper internals comprise discontinuous guide elements consisting of plane plates, called guide plates pierced with openings and continuous guide means consisting of sheaths and slit tubes extending in the vertical direction of the tube, the openings of the plates and the bores of the continuous guide means being in positions corresponding to the positions of the absorber rods in the bundle of the control rod.
The openings passing through the guide plates and the bores of the sheaths and of the slit tubes constituting the guide elements for the absorber rods have lateral slots putting them in communication with a free space left in the central part of the guide tube and allowing passage of a spider body for assembling the cluster of parallel absorber rods.
In order to control the nuclear reactor by adjusting the reactivity of the core, the clusters of absorber rods are displaced in the vertical direction, during operation of the reactor, so that the absorber rods are introduced over a greater or lesser height inside certain assemblies of the core. During their displacement in the vertical direction, the absorber rods of the control rods are each guided by a series of aligned openings in the guide plates and by the internal bore of a guide sheath or of a split tube.
After a certain period of operation of the nuclear reactor, the openings in the guide plates and the bores in the sheaths and in the split tubes may exhibit wear, manifested for example by ovalization of the cross-section of the opening or of the bore, so that the absorber rods are no longer guided completely satisfactorily.
It is therefore necessary to carry out periodic checks and measurements on the guide elements for the upper internals of the nuclear reactor, so as to determine whether some of these guide elements need to be replaced or repaired.
Checking the guide elements must be carried out during a period of shutdown for checking, repair and/or reloading the nuclear reactor, during which period the upper internals are withdrawn from the vessel and laid down in the bottom of the pool of the reactor, on a storage and intervention stand.
It is necessary to carry out extremely precise measurements on the guide elements for the upper internals, insofar as wear of these guide elements requiring repair or replacement is manifested by small differences between the dimensions of the openings or bores of the guide elements in various transverse directions.
In French Patent Application FR-A-89-04082 filed by Framatome, it was proposed to carry out these checks on the basis of comparative capacitance measurements of capacitors constituted by armatures distributed about the axis of a sensor which is displaced in a substantially coaxial manner inside the guide elements.
This device, which makes it possible to perform rapid measurements over the entire length of the guide elements may, however, lack precision and requires complex elements for gathering and using the measurements.